Single wavelength semiconductor laser with grating-assisted dielectric waveguide coupler

ABSTRACT

A grating ( 18 ) couples the waveguide region ( 36 ) of a semiconductor laser ( 11 ) to a dielectric waveguide ( 26 ). The waveguide region of the laser includes a mirror ( 15 ) at one end thereof and an absorber ( 19 ) at the other end thereof. The dielectric waveguide includes a reflector ( 24 ) therein to reflect a portion of the light coupled from the laser to the dielectric waveguide back into the laser waveguide region.

This application claims priority under 35 USC 119(e)(1) of provisionalapplication No. 60/069,489 filed Dec. 15, 1997.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to a single wavelength laser withgrating-assisted dielectric waveguide coupler.

2. Brief Description of the Prior Art

Optical communication systems typically employ semiconductor lasersources and glass optical fiber communication channels. There are manyconfigurations of semiconductor lasers including various materialcompositions and various dimensions of the grown layers that form theactive region and an associated optical waveguide in the laserstructure. The material composition of the active region determines thewavelength of operation. For example, at lasing wavelengths of about 0.9μm, the group III-V materials of the ternary compound Al_(x)Ga_((1−x))Aswith GaAs quantum wells provide a compact and rugged source of infraredlight which can be easily modulated by varying the diode current.Communications systems of this type are discussed in Ser. No.08/248,937, now issued as U.S. Pat. No. 6,064,783, the contents of whichare incorporated herein by reference. Light from a laser can beextracted by abutting an optical fiber thereto in known manner, however,devices fabricated in this manner do not lend themselves tosemiconductor fabrication. Lasers can be abutted to optical fibers,however the indices of refraction between optical fibers andsemiconductor material are so dissimilar that the amount of coupling isvery low, leading to an inefficient device. Furthermore, the alignmentof the source with an optical fiber is quite tedious when high couplingefficiency is desired. This mismatch of the light field of the laser andthat of the optical fiber also affects the amount of light coupled tothe fiber.

In the device described in the above noted application, the directcoupling of the semiconductor laser output into an optical fiber wasimproved over the prior art by providing a semiconductor laserintegrated with a silicon dioxide based waveguide having high efficiencycoupling of the laser output into the waveguide by an integrated gratingto permit the laser output to be coupled into an optical fiber by buttcoupling of the optical fiber to the silicon dioxide based waveguide.The grating, when appropriately designed as discussed in the above notedapplication, provides a matching of the propagation in the laser withthe propagation in the glass. The period of the grating determines thewavelength of that portion of the light in the laser waveguide that willbe passed through the grating to the optical fiber. Multiple lasers withdifferent wavelengths could be integrated and their outputs coupled andcombined into a single waveguide for wavelength division multiplexedoperation. A problem with the device of the above-mentioned applicationis that the narrow bandwidth of grating assisted directional couplersmakes it difficult to match them with an integrated single wavelengthlaser source whose lasing wavelength must lie within the bandwidth ofthe coupler. The architecture of grating assisted directional couplerstypically consists of two waveguides and an optical grating whose periodis dictated by the geometry of the two waveguides. The geometricalproperties include the waveguide dimensions as well as the refractiveindex profiles. Generally, the lasing wavelength is governed by anoptical grating that produces the necessary feedback to the laser.Precise machining of both gratings must be made to allow forsatisfactory operation.

SUMMARY OF THE INVENTION

The above noted problems of the prior art are minimized in accordancewith the present invention.

Briefly, there is provided a semiconductor laser, preferably of the typeset forth in the above noted copending application, coupled on one sideof the anode/cathode structure of the laser to a dielectric waveguidevia a grating as in the above noted application. The semiconductor laseris formed from either group III-V compound materials or from group II-VIcompound materials (the material system determines the wavelength ofoperation), and preferably includes various hetero-junctions and thinlayers that form quantum well regions. Generally, there are twohetero-junctions that form the boundary between a central, highrefractive index (relative to the central region). The light generatedfrom the active region (generally part of the central region) is thenconfined by the high refractive index layer (central region). Thecoupling grating is formed at the interface of the laser cladding layerand the cladding layer of the dielectric waveguide. The dielectricwaveguide is formed from silicon dioxide based materials and preferablyphosphosilicate glass which is doped p-type having from about 8 to about10 percent by weight of dopant. The dielectric waveguide is preferablyphosphosilicate glass with the dopant being phosphorous. The dielectricwaveguide includes a reflector, such as, for example, mirror, at an endregion formed by high reflection coating the dielectric waveguide facetthereof to reflect a portion of the light therein back through thegrating and into the laser to provide the required feedback of thedesired light frequency as determined by the grating while all otherfrequencies which are not passed through the grating are absorbed by theabsorber and are not reflected back into the laser active region. Thelight fed back through the grating will be at the same wavelength as thelight initially transmitted through the grating. It follows that thegrating is performing the function of both reflection and wavelengthselection. In this manner, the desired lasing wavelength is enhanced bythe light being fed back whereas all other light wavelengths arerejected or minimized by being absorbed by the absorber.

In operation, broad band light (stimulated emission) is generated in thelaser active region. The generated light from the active regionpropagates to both the front (right direction) and to the rear (leftdirection). Light traveling in the left (backward direction) isreflected by a broad band reflector (mirror) that may be located ateither the edge of the active region or at a distance from the edge. Thebackward propagating light beam is reflected back into the active regionand produces the total beam that is propagating to the right (forwarddirection). In the absence of the grating coupler, the light travelingout of the front of the active region continues in the semiconductorwaveguide section and enters into an absorbing region. As a result,light is not fed back into the active region and, accordingly, thestimulated emission spectrum will not become narrow (i.e., non lasingcondition). In the presence of the grating and an auxiliary waveguidethat is parallel to the semiconductor waveguide (co-planar geometry),the broad band light within the laser waveguide is coupled (andfiltered) to the auxiliary waveguide. The filtered-out wavelengthcoupled to the auxiliary waveguide is determined by the grating period,grating depth, relative location, and the geometrical and dielectriccharacteristics of both the semiconductor and auxiliary waveguide. Areflector, which is formed by a high reflection coating on thedielectric facet in the auxiliary waveguide, such as, for example, amirror, is provided to reflect a portion of the light in the auxiliarywaveguide back into the semiconductor waveguide. Since the lightreflected by the mirror in the auxiliary waveguide has the specificwavelength (or narrow-band spectrum that was initially filtered by thegrating), the light will naturally couple back to the semiconductorwaveguide and, accordingly, be fed back into the laser active region.The result is that the stimulated emission within the laser is enhancedonly at the wavelength transmitted through and fed back by the grating.All other wavelengths generated from the active region of the laser areabsorbed by the absorber at the terminal region of the laser.

While only a single laser is discussed herein, it should be understoodthat the device in accordance with the present invention can replace oneor all of the devices 410, 420, 430 or 440 in FIG. 4a of the above notedapplication with the grating in each device being adjusted to thedesired wavelength.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a cross-section through the laser, grating and waveguidein accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the FIGURE, there is shown a laser 10 having an activeregion location 11 and the associated laser waveguide core region (31)beyond the region under the anode (active layer region). The P-typeanode 12 and the N-type cathode 13 are disposed on opposing sides of theactive region. The laser 10 is disposed on the semiconductor substrate16. The laser includes a reflector 15 in the form preferably of a mirroron the back side of the laser waveguide/cladding layers. Electricalcurrent flowing through the diode causes electron/hole recombination inthe active area (holed from anode 12 and electrons from the cathode 13)which releases energy as photons. Outside the active area 11 the laserwaveguide core 31 and cladding 27, 32 extend from the active region. Anabsorber 19 is disposed in the laser waveguide core layers beyond thegrating 18. The semiconductor waveguide 36 confines generated photons inthe active region to the high index of refraction region of the laserwaveguide core 31.

A coupling grating 18 is disposed adjacent the anode 12 and on the sidethereof remote from the mirror 15. The silicon dioxide layer 17 extendsover the grating 18. Light having a wavelength as determined by theperiod of the grating 18 is passed through the grating 18 to thephosphosilicate glass core 21. PSG core 21 is doped with 8 percent byweight phosphorous and has an index of refraction of 1.46. PSG core 21has a cladding thereover of silicon dioxide 22. SiO₂ layer 17, glasscore 21, and cladding 22 cooperate to form dielectric waveguide 26. Thelight passes along the glass core 21 with some of the light passingthrough to a standard core 25 for external transmission and a portion ofthe light striling the mirror 24 and being reflected therefrom backthrough the core 21 and the axial region containing the grating 18 toprovide the feedback to the laser. The light which is not passed throughthe rating 18 is absorbed by the absorber 19. Mirror 24 is formed byhigh reflection coating of the waveguide 21 facet. Mirror 15 is formedby high reflection coating the laser waveguide facet.

Though the invention has been described with reference to a specificpreferred embodiment thereof, many variations and modifications willimmediately become apparent to those skilled in the art. It is thereforethe intention that the appended claims be interpreted as broadly aspossible in view of the prior art to include all such variations andmodifications.

What is claimed is:
 1. An integrated circuit comprising: a semiconductorlaser, wherein said semiconductor laser includes an active region, asemiconductor waveguide region, and a first reflector; a dielectricwaveguide coupled to said semiconductor laser by a grating; and a secondreflector in said dielectric waveguide to feed back light front saiddielectric waveguide to said semiconductor laser; wherein said firstreflector is disposed at a region of said semiconductor waveguide regionremote from said dielectric waveguide, thereby feeding back lightthrough said active region toward said dielectric waveguide; and saidsemiconductor laser does not include a reflector disposed at a region ofsaid semiconductor waveguide region proximate said dielectric waveguide.2. The circuit of claim 1 wherein said semiconductor waveguide regionfurther including an absorber therein.
 3. The circuit of claim 2 whereinsaid absorber is disposed at one end region of said waveguide regionremote from said first reflector.
 4. A method of forming an integratedcircuit which comprises the steps of: (a) providing a semiconductorlaser having an active region and a semiconductor waveguide region; (b)disposing a first reflector at one terminal region of said semiconductorwaveguide region and an absorber in said semiconductor waveguide regionremote from said reflector, without disposing another reflector betweensaid active region and said absorber; (c) providing a dielectricwaveguide; and (d) coupling said dielectric waveguide to saidsemiconductor waveguide with a grating, said grating disposed proximatethe semiconductor waveguide region between the absorber and the activeregion.
 5. The method of claim 4 further including the step of disposinga second reflector in said dielectric waveguide to provide feedback fromsaid dielectric waveguide to said semiconductor waveguide.